Just came back from South America for the holidays. T-Mobile One plans include unlimited international coverage at up to 2G speed. I thought I'd post here my experience at the various cities so others could plan for their trips accordingly.

Not having to visit a local shop for SIM cards or worrying about topping up plans was invaluable to me. I did have to restart my phone more times than I'd like to to get my phone to register on the network. But overall, I was pretty impressed with T-Mobile, I'd say. For the record, I was mostly using the app for Uber, Google Map, WhatsApp, quick email checking, and some light web browsing. If you are hoping to be able to do video / audio streaming, you will be disappointed.

* Bogota, Colombia - way faster than the 2G speed T-Mobile advertised. Not a problem for me here
* Buenos Aires, Mar del Plata & Rosario, Argentina - closer to 2G speed. Had to reboot phone quite a few times during my visit
* Montevideo, Uruguay - Roaming did not work here for me at all. My phone would register on the network (shows Claro Uruguay) but I couldn't connect to any internet. Montevideo is a small city so with some planning, it's doable to visit the city for a short stay with no phone data. I relied on hotel / restaurant / coffee shop wifi. Would have liked my data to work on my phone but oh well, I made the best out of what I could manage

Hope this helps someone.